Rebecca MacKinnon has an important piece in the Wall Street Journal today about China's "Green Dam Youth Escort" filtering mandate and the danger of this model catching on with other governments. "More and more governments -- including democracies like Britain, Australia and Germany -- are trying to control public behavior online, especially by exerting pressure on Internet service providers," she notes. "Green Dam has only exposed the next frontier in these efforts: the personal computer."

She's right, and that's cause for serious concern. Moreover, there's the question of how corporations doing business in China should respond to demands and threats related to installing such filters. She notes:

In a world that includes child pornographers and violent hate groups, it is probably not reasonable to oppose all censorship in all situations. But if technical censorship systems are to be put in place, they must be sufficiently transparent and accountable so that they do not become opaque extensions of incumbent power -- or get hijacked by politically influential interest groups without the public knowing exactly what is going on.

Which brings us back to companies: the ones that build and run Internet and telecoms networks, host and publish speech, and that now make devices via which citizens can go online and create more speech. Companies have a duty as global citizens to do all they can to protect users' universally recognized right to free expression, and to avoid becoming opaque extensions of incumbent power -- be it in China or Britain.